DEVICES WHICH USE LIGHT 



113 



shade of blue or brown. The opening in the iris is 

 called the pupil, the size of which changes according 

 to the intensity of light. The iris has in it two sets of 

 muscles that regulate the size of the pupil. If you 

 stand in front of a mirror in a dark room for several 

 minutes and then turn on the lights you can see the 

 pupils of your eyes change in size. Directly back of 

 the iris is a convex lens. It is made of a soft, trans- 

 parent substance, and its shape" is changed by muscles 



RETINA 



CORNEA 



IRIS 



FIG. 175. CROSS SECTION OF THE EYE 



attached to it. These accommodate the eye to changes 

 in distance from the objects we look at. The large 

 space between the lens and the retina is filled with a 

 jelly-like, transparent substance called vitreous humor. 

 This substance gives the eye its shape. The retina is 

 connected to the optic nerve which leads to the brain. 



With the exception of bodies such as the sun, the 

 stars, a candle, or an electric light, which shine by 

 their own light, we see all objects by reflected light. 

 The reflected light rays from an object enter the 

 pupil of the eye and pass through the double convex 

 lens, which bends them so that an image is formed 

 on the retina. This affects the optic nerve, which car- 

 ries the sensation to the brain, and the brain cells 

 interpret the message that it receives. 



The statement was made in the preceding para- 

 graph that the eye is built to accommodate for dif- 

 ferences in distance from objects which we look at. 

 That is, we can see near or distant objects. Do you 

 know how it is done? Look at Figure 173. It shows 

 that the distance of the image from the lens varies 

 with the thickness of the lens. The thicker the lens, 

 the nearer the image is to it. The distance of the im- 

 age from the lens also changes as the distance of 

 the object from the lens changes. The farther the 

 object is from the lens, the nearer the image is to it. 

 Since the retina, which receives the images in the 

 eye, cannot move, it is always the same distance from 



the lens. The eye is so constructed, however, that it 

 can change the shape of the lens. When objects are 

 near, the lens becomes more rounded and bends the 

 light rays at a greater angle, focusing them on the 

 retina. When we are looking at objects far away the 

 lens becomes thinner so that the rays of light can be 

 focused on the retina without being bent so much. 



Of what importance is the proper care of the eyes? 

 The eyes are delicate organs and should be cared for 

 as such. The loss of sight is one of the greatest trag- 

 edies of life. 



The eye is naturally an organ of long-range vision, 

 but many people, especially students, must do much 

 close work. When we look at near objects, such as 

 a book, muscles of the eye contract in order to make 

 the lens thicker. This keeps the muscles of the eye- 

 ball under strain and they become tired just as other 

 muscles of the body become tired. When one has to 

 do a great deal of reading, it is a good thing to relax 

 the eye muscles occasionally by looking at some dis- 

 tant ob'ect or by closing the eyes for a short time. 



Object 



Lens Iroage Object Lens 



Object 'Lens Image Object 'Lens Irrwge 



FIG. 176. HOW THE EYE ADJUSTS FOCUS 



Never read or study while facing a well lighted 

 window or lamp. It is best to have the light for read- 

 ing and writing always coming from behind you over 

 your shoulders. The page that one is reading should 

 always be well lighted without glare. 



Infections of the eyeball and lids are common, 

 especially with children. They are known by various 

 names such as sore eyes, pink eye, a sty, etc. For 

 mild infections a concentrated solution of boric acid 

 usually proves effective. It is always best, however, 

 to be examined by an eye specialist and have him pre- 

 scribe a remedy. By all means do not purchase patent 

 medicines for the eye. Avoid them and consult a re- 

 liable physician. 



How are the eye defects, nearsightedness, farsight- 

 edness, and astigmatism, corrected? The normal eye 

 accommodates itself for distance so that the images 

 of objects always fall on the retina in such a manner 

 as to produce clear vision. Unfortunately many per- 

 sons have imperfect eyes so that eyeglasses must be 

 worn to help the eyes do their work properly. The 



